Measurable residual disease does not preclude prolonged progression-free survival in CLL treated with ibrutinib
Wang XV, Hanson CA, Tschumper RC, et al. Measurable residual disease does not preclude prolonged progression-free survival in CLL treated with ibrutinib. Blood. 2021;138(26):2810-2827. doi:10.1182/blood.2020010146
E1912 was a randomized phase 3 trial comparing indefinite ibrutinib plus six cycles of rituximab (IR) to six cycles of fludarabine, cyclophosphamide and rituximab (FCR) in untreated younger patients with CLL. We describe measurable residual disease (MRD) levels in E1912 over time and correlate them with clinical outcome. Undetectable MRD rates (< 1 CLL cell per 104 leukocytes) were 29.1%, 30.3%, 23.4% and 8.6% at 3, 12, 24 and 36 months for FCR, and significantly lower at 7.9%, 4.2% and 3.7% at 12, 24 and 36 months for IR, respectively. Undetectable MRD at 3, 12, 24 and 36 months was associated with longer progression-free survival (PFS) for the FCR arm with hazard ratios (MRD detectable / MRD undetectable) of 4.29 (95% CI 1.89 - 9.71), 3.91 (95% CI 1.39 - 11.03), 14.12 (95% CI 1.78 - 111.73), and not estimable (no events among those with undetectable MRD), respectively. For the IR arm, patients with detectable MRD did not have significantly worse PFS compared to those in whom MRD was undetectable; however, PFS was longer for those with MRD levels of less than 10-1 compared to those with MRD levels above this threshold. Our observations provide additional support for the use of MRD as a surrogate endpoint for PFS in patients receiving FCR. For patients on indefinite ibrutinib-based therapy, PFS did not differ significantly by undetectable MRD status, while those with MRD less than 10-1 tend to have longer PFS, although continuation of ibrutinib is very likely required to maintain treatment efficacy.